1. Field of the Invention
The present invention relates to an optical writing device, an image forming apparatus, and a correction value information generating method.
2. Description of the Related Art
Recently, electronization of information has been encouraged, and image processing apparatuses such as printers or facsimiles used for outputting electronic information and scanners used for electronizing documents play essential roles accordingly. Such image processing apparatuses are often configured as multifunction peripherals (MFP) that can be used as a printer, a facsimile, a scanner, and a copying machine, by being provided with an image capturing function, an image forming function, and a communication function, for example.
Among such image processing apparatuses, electrophotographic image forming apparatuses are widely used as image forming apparatuses for outputting electronic documents. An electrophotographic image forming apparatus outputs an electronic document to a paper sheet by exposing the photosensitive element to form an electrostatic latent image, developing the electrostatic latent image into a toner image with a developer such as toner, and transferring the toner image onto the sheet.
In the electrophotographic image forming apparatus, available as optical writing devices used for exposing the photosensitive drum include a laser diode (LD) raster optical system and a light emitting diode (LED) writing system. A device of the LED writing system includes an LED array (LEDA) head.
The optical writing device of the LED writing system forms an electrostatic latent image by exposing the photosensitive drum with the LEDA as mentioned earlier. If the distance between the LEDA and the photosensitive drum changes, the spot diameter of the beams output from the LEDA and reaching the photosensitive drum also changes. As a result, an image density variation occurs.
For example, when the photosensitive drum is decentered or when the film thickness varies across the entire surface of the photosensitive drum, the distance between the photosensitive drum and the LEDA changes as the photosensitive drum is rotated. This results in a density variation along the sub-scanning direction in a formed image.
To address this issue, some technologies have been developed to keep the distance between the photosensitive drum and the light source constant (for example, see Japanese Patent Application Laid-open No. 2010-008913, Japanese Patent Application Laid-open No. 2006-187929, and Japanese Patent Application Laid-open No. H7-052447). Technologies for correcting a periodic variation caused by rotation of the photosensitive drum have been also developed (for example, see Japanese Patent Application Laid-open No. 2007-144731).
Using the technologies disclosed in Japanese Patent Application Laid-open No. 2010-008913, Japanese Patent Application Laid-open No. 2006-187929, and Japanese Patent Application Laid-open No. H7-52447 requires components for keeping the distance between the photosensitive drum and the light source constant. The arrangement of the components could be complex, resulting in an increase in apparatus and management costs and reduced productivity.
The technology disclosed in Japanese Patent Application Laid-open No. 2007-144731 can address an image quality variation caused by a relative speed variation of the surface of the photosensitive drum with respect to the light source because of a variation of the distance between the photosensitive drum and the light source.
Solely adjusting the light emission cycle of the light source, however, cannot address an image quality variation caused by a varying beam spot diameter or varying beam intensity because of a varying distance between the surface of the photosensitive drum and the light source.
In response to this issue, if the distance between the surface of the photosensitive drum and the light source is known across the entire circumferential surface of the photosensitive drum in the rotating direction, a correction can be made corresponding to the distance. The distance between the surface of the photosensitive drum and the light source, however, varies depending on how components are assembled within the apparatus. Apparatuses of the same model may have different distances, and such distances need to be obtained for individual apparatuses. Furthermore, the distance between the surface of the photosensitive drum and the light source could also change depending on how much the photosensitive drum are worn out, for example, by operations of the apparatus. Therefore, it is not realistic to manually obtain the distance between the surface of the photosensitive drum and the light source.
Therefore, there is a need for technique capable of obtaining a correction value for addressing a variation of the distance between a photosensitive drum and a light source with a simple structure.